1. Field of the Invention
This invention relates to a new channel design to reduce impact ionization in heterostructure field-effect transistors (HFETs).
2. Description of the Related Art
Future generations of microwave and millimeter-wave radar, communications, electronic warfare, smart weapons, and imaging systems will require higher precision, smaller size, increased bandwidth, lower operating voltages, and lower cost of production. To meet the demand for improved high-frequency performance, considerable effort within the past ten years has focused on the development of GaAs-based and InP-based high-electron-mobility transistors (HEMTs).
As a result, a variety of HEMT circuits have been fabricated which operate at higher frequencies and have improved power, efficiency, gain, and noise figure performance. The primary factors responsible for the improved HEMT performance have been the increase in the In mole fraction in the In.sub.x Ga.sub.1-x As channel and the increase in the conduction band offset at the 2DEG interface. As a result of these improvements, InP-based HEMTs have distinct millimeter-wave performance advantages compared to GaAs-based HEMTs, and currently hold the record in frequency response and noise figure for any FET.
In the longer term, AlSb/InAs-based HEMTs may be more attractive than InP-based HEMTs for some of the above applications due to the substantially improved material properties of this heterojunction system. Higher electron mobility and higher electron velocity can be achieved with an InAs channel compared to In.sub.x Ga.sub.1-x channels. The lower electron effective mass of InAs gives this material system a significant advantage in the room-temperature mobility which can be achieved for a given HEMT sheet charge density. Due to the large .GAMMA.-L valley separation, InAs also has the substantial advantage of a higher electron peak velocity (4.times.10.sup.7 cm/sec) compared to the other semiconductors. The considerably larger conduction band discontinuity(.DELTA.E.sub.c =1.35 eV) at the donor layer/channel interface enables the formation of a deeper quantum well and the associated benefits of a larger two dimensional electron gas (2DEG) sheet charge density, superior carrier confinement, and improved modulation efficiency. These features should enable improved scaling of the current-gain cutoff frequency(f.sub.T) as the gate length is reduced to the nanometer range.
In addition to the increased high-frequency performance potential, InAs-channel HEMTs are also attractive for applications requiring low-voltage operation. The higher electron mobility and velocity, lower threshold field, and reduced access resistance capability enable the attainment of higher effective velocity at a significantly lower drain voltage.
Although improvements have been made in recent years, the material growth and fabrication technology for AlSb/InAs HFETs is relatively immature. The high reactivity of AlSb in air and the low valence-band offset of the AlSb/InAs heterojunction increase the complexity of the material growth and device design requirements. AlSb/InAs-based HFETs have previously been fabricated with barrier layers which include AlSb, AlGaSb, AlSbAs, AlGaSbAs, and a superlattice consisting of AlSb/AlAs. Each of these approaches has advantages and disadvantages relating to growth complexity, stability, conduction band and valence band offset, and insulator effectiveness.
Reduction of the impact ionization in the devices, however, has yet to be addressed. The HEMTs are susceptible to significant charge control problems associated with impact ionization in the InAs channel due to its narrow bandgap of 0.36 eV at room temperature. These effects become increasingly pronounced as the gate length is reduced due to the higher fields present, thus hindering the high-frequency performance of short-gate length HEMTs and limiting their operating voltage range.
We have found that the large 1.35 eV conduction band offset of the AlSb/InAs heterojunction offers unique opportunities to reduce impact ionization effects by using channel layer designs that exploit quantum confinement in this region. The dc and microwave characteristics of 0.1 .mu.m AlSb/InAs HEMTs exhibit reduced impact ionization and improved charge control due to the addition of a thin InAs subchannel in the HEMT design.
3. Objects of the Invention
It is an object of this invention to provide improved heterostructure field-effect transistors (HFETs) to reduce impact ionization.
It is a further object of this invention to provide improved HFETs to exhibit higher transconductance.
It is a further object of this invention to provide improved HFETs to exhibit lower output conductance.
It is a further object of this invention to provide improved HFETs to exhibit reduced gate leakage current.
It is a further object of this invention to provide improved HFETs to exhibit higher operating drain voltage.
It is a further object of this invention to provide improved HFETs to exhibit improved frequency performance.
It is a further object of this invention to provide improved HFETs to improve voltage gain.
It is a further object of this invention to provide improved HFETs to improve power gain.
It is a further object of this invention to provide improved HFETs by adding the design flexibility gained by being able to tune the ground state of the main channel and a new sub-channel by choosing the proper thickness of the respective quantum wells.
These and further objects of the invention will become apparent as the description of the invention proceeds.